Among the prior patents, two may be singled out, these being U.S. Pat. No. 2,101,257, issued Dec. 7, 1937 to M. Vogel-Jorgensen, for "Apparatus for Measuring Liquid or Fluent Materials", and U.S. Pat. No. 2,392,951, issued Jan. 14, 1946 to R. G. Salisbury, and entitled "Flow Meter". In the case of both of these patents, the concept is to provide a calibrated volume which may be switched into and out of the normal flow line for the liquid, allowing the flow rate to be monitored at timed intervals. A drawback of these prior art patents is that the calibrated volumes provide only an approximate idea of the total flow, and are incapable of exact precision.
My own earlier U.S. Pat. No. 4,455,870, issued Jun. 26, 1984 and entitled "Method and Apparatus for Determining Liquid Flow Rates" attempts to solve the problems inherent in the prior art mentioned above. My prior patent provides a method for determining the total inflow of a liquid through a liquid-flow system in which the liquid enters a sump cavity and is pumped out of the sump cavity by pump means. A computer calculates an on-going total inflow volume for the liquid by (1) adding in the sump cavity volume between lower and upper limit levels each time the liquid surface rises to the upper limit level, and (2) determining the inflow rate over the last portion of the filling time just described and extrapolating this inflow rate over the time during which the pump means is operating, to yield an incremental quantity, such quantity being added into the ongoing total inflow volume.
The said earlier patent provided a satisfactory procedure for determining total inflow volume in a relatively reliable way, but in certain cases it was not as accurate as desired, especially when the inflow rate was changing more or less dramatically during the pumping cycle.
The latter problem was addressed by my U.S. Pat. No. 4,669,308, issued Jun. 2, 1987, and entitled "Method and Apparatus for Determining Liquid Flow Rates". In this second patent, improved accuracy was obtained, but at the cost of requiring a significantly more complex apparatus. In brief, the solution offered by the latter patent was to calculate an inflow rate during a pumping phase by determining the inflow rate during two sequential intervals just prior to the pumping phase, and over two sequential intervals just after the pumping phase. For each pair of intervals, the flow rates were extrapolated either forward or backward (as the case may be) for the pumping phase in question, and the two extrapolated values were averaged before being multiplied by the duration of the pumping phase in order to arrive at a total volume entering during the pumping phase.
My further prior U.S. Pat. No. 5,182,951, issued Feb. 2, 1993, and entitled "Method and Apparatus for Calculating Flow Rates Through a Pumping Station" uses a different approach to calculating total flow volume. The apparatus includes at least one pump, and also includes, for each pump and for each combination of pumps, an oscillator circuit of which the output frequency can be adjusted. The frequency of each such circuit is adjusted to represent the pump rate for the pump or combination of pumps to which that oscillator circuit corresponds. While operating the pumping station, a totalizer has fed to it the output frequency of that oscillator circuit which corresponds to the pump or combination of pumps that is operating in any given time, and the total in the totalizer is incremented by a given amount for each pulse encountered, thus continuously updating the totalizer.
My further U.S. Pat. No. 5,190,442, issued Mar. 2, 1993, and entitled "Electronic Pumpcontrol System", is an attempt to achieve greater sophistication and accuracy in a system incorporating a plurality of pumps which can be operated individually or in various combinations. In view of the fact that most pumps produce a flow rate which varies depending upon backpressure, the inventive combination set forth in my latter patent senses backpressure and includes an addressable memory for storing pumping rate values vs. backpressure for each of the pumps. A processor is programmed such that, on high backpressure, the pump controller avoids pump starts that will not result in a net increase in the total pumping rate, whereas on a decrease in backpressure, the pump controller allows more pumps to start when called for, and allows for the starting of more pumps than the minimum necessary, in order to decrease the duration of pumping. The pump controller avoids a pump start under conditions which would result in the pump undergoing rapid on/off cycling, and further the pump controller avoids starting and stopping the pumps on pressure surges.
Summarizing the prior art, it can be said that total liquid flow determination can be relatively accurately determined, but the accuracy comes at a substantial cost due to complexity. By contrast, the simpler arrangements generally fail to provide the kind of accuracy that is expected from a monitoring system of this kind.